Although balancer coils are generally used for hot-cathode florescent lamps and known, they were initially intended for low-voltage discharge lamps and were not small-sized.
Since the balancer coil is required to be small for cold-cathode florescent lamps and the drive voltage thereof is high, parasitic capacitance should be taken into consideration, which does not require consideration for hot-cathode florescent lamps.
For the use of discharge lamps such as cold-cathode florescent lamps, which require a high voltage and high impedance, it is important to consider not only the parasitic capacitance around the wiring connecting with a cold-cathode florescent lamp, but also the parasitic capacitance between windings of a balancer coil.
A plurality of solutions about a balancer coil has been proposed for the use of cold-cathode florescent lamps. However, each of the solutions contains an extreme instability, and the practical feasibility for the use of cold-cathode florescent lamps is uncertain.
One of the main reasons is the fact that the effects of a balancer coil are unstable. Another is the fact that a balancer coil has not been realized to have a small and thin form to meet the market demands even though the coil has stable effects.
Representative examples regarding balancer coils, as the understanding among those skilled in the art, include Japanese Laid-Open Patent Publication (Kokai) No. Hei 7-45393 (Japanese Patent No. 3291852).
FIG. 4 in the prior art publication corresponding to FIG. 18 discloses that it is important to make the inductance difference between coils N1, N2 in a balancer coil of a cold-cathode florescent lamp smaller. Taking FIG. 4 in the prior art publication as an example, the smaller inductance difference is intended to achieve by winding the two coils alternately.
The structure of the example disclosed in the prior art also aims to make coupling coefficient higher, and those skilled in the art understand that it is important for a balancer coil for cold-cathode florescent lamps to have a high coupling coefficient.
In order to make the coupling coefficient between coils higher, therefore, a balancer coil is ideally a generally rectangular parallelepiped.
For example, FIG. 19 is one example of the prior art of the balancer coil for cold-cathode florescent lamps, which has been considered as the smallest, except the balancer coil disclosed by the inventors of the present invention in the specification of Japanese Patent Application No. 2004-3740 (U.S. Patent application No. 2004-155596).
In the prior art, each coil of a balancer coil has no sections, is layer-wound, and is made into a generally rectangular parallelepiped shape so as to make the coupling coefficient higher.
That is, since the prior art is based on the technical idea that making coupling coefficient higher is important, a flat shape is avoided. The section structure for a coil is similarly avoided since the coupling coefficient decreases thereby.
Also, the balancer coil has to be layer-wound so as not to decrease the coupling coefficient.
The prior art of parallel driving cold-cathode florescent lamps for multiple lamps is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2003-31383, in addition to the specification of Japanese Patent Application No. 2004-3740 (U.S. Patent application No. 2004-155596) invented by the present inventors.
FIG. 4 of the specification of Japanese Patent Application No. 2004-3740 (U.S. Patent application No. 2004-155596) corresponding to FIG. 20 discloses multiple balancer coils connected circulatingly. FIG. 6 of the specification of Japanese Patent Application No. 2004-3740 (U.S. Patent application No. 2004-155596) corresponding to FIG. 21 discloses the technique of making magnetic fluxes of three or more coils face one another.
FIG. 6 of Japanese Laid-Open Patent Publication (Kokai) No. 2003-31383 corresponding to FIG. 22 discloses windings W1-Wn wound around the same core, whose numbers of turns are equal.
A balancer coil is easily applicable to a hot-cathode florescent lamp, which is mainly because a hot-cathode florescent lamp can be driven with a low voltage and low impedance. A hot-cathode florescent lamp does not particularly have to be small for its application. From that standpoint, a balancer coil may be large. The inductance of a balancer coil compared to the impedance of a hot-cathode florescent lamp (or the reactance at the operational frequency of an inverter circuit) becomes large enough without any particular consideration, thereby bringing full performance as a balancer.
For the application of a cold-cathode florescent lamp, however, since the drive voltage and impedance of a cold-cathode florescent lamp are high, the reactance required for a balancer is also large. In addition, the influence of the parasitic capacitance produced in each high-voltage part or on windings cannot be ignored.
Since a balancer is mainly used for precision instruments including a backlight for a liquid crystal display television, it is required to be a small or flat shape.
When either of the cold-cathode florescent lamps connected to a balancer coil is not lighted, the core of the balancer coil is saturated, thereby increasing the core loss to elevate temperature. In order to prevent the temperature from rising, the core has to be small as disclosed in the specification of Japanese Patent Application No. 2004-3740 (U.S. Patent application No. 2004-155596).
Those skilled in the art do not necessarily understand correctly the technique of a balancer.
Its typical example, as disclosed in the specification of Japanese Patent Application No. 2004-3740 (U.S. Patent Application No. 2004-155596) invented by the inventors of the present invention, lies in the fact that those skilled in the art make an excessive setting of the reactance required for a balancer to be several times as large as or larger than the integrated impedance of a cold-cathode florescent lamp in the prior art. Regarding this, it is essential that a negative resistance characteristic, which is the differentiated impedance of a cold-cathode florescent lamp, exceed the sum of the reactances of a balancer. It is disclosed that a shunt characteristic is secured by controlling/measuring an impedance characteristic when integrating a cold-cathode florescent lamp into a backlight.
As is seen from Japanese Patent No. 3291852, however, the technical idea persists that coupling coefficient and uniformity in a winding parameter are required as the main parameter showing the effect of a shunt/balance characteristic, and it is the conventional knowledge that there are many restrictions in designing a balancer coil.
A balancer coil, therefore, cannot have a section structure, and has to be a generally rectangular parallelepiped so as to increase coupling coefficient.
It is difficult for the example as disclosed in the publication of Japanese Patent No. 3291852 and many prior arts to have a voltage breakdown structure. It is also difficult for FIG. 23 corresponding to FIG. 4 disclosed in the specification of Japanese Patent Application No. 2004-79571 by the inventors of the present invention to achieve a balancer coil of high voltage breakdown.
Recently, however, the disclosure of the specification of Chinese (TAIWAN) Patent No. 521947 has made clear that it is not coupling coefficient but mutual inductance which is important for a balancer coil.
In a balancer coil for cold-cathode florescent lamps, the impedance and negative resistance of the cold-cathode florescent lamp are considerably larger even compared with those of a hot-cathode florescent lamp, and a very large mutual inductance is required.
Therefore, a balancer coil for cold-cathode florescent lamps has to be wound up by multiple very thin wires. In this respect, the parasitic capacitance generated in a winding lamp (so-called distributed capacitance) cannot be ignored.
It is known, as self-resonance, that resonance takes place between the parasitic capacitance between windings and the self-inductance of windings.
For a balancer coil, when the self-resonance frequency becomes lower than the frequency used in the balancer coil, the balancer coil loses its shunt characteristic and balance characteristic. However, it is difficult to say that such finding has been known among those skilled in the art and no such finding has been disclosed in the prior art.
No such a point has been disclosed in the prior art relating to a balancer coil for cold-cathode florescent lamps. Most of the reasons that a balancer coil for cold-cathode florescent lamps is too unstable for practical use are originated from excessive winding for securing mutual inductance.
Specifically, this is because excessive winding leads to too low self-resonance frequency of the balancer coil, which results in shunt/balance effect has been lost. That is, a balancer coil for cold-cathode florescent lamps has an appropriate range of the number of turns relative to the characteristic of a cold-cathode florescent lamp. Thus, when exceeding the range and when not reaching the range, the shunt/balance effects are lost.
It is known, as the general knowledge, that effective magnetic permeability becomes high when a core is large.
Since a large inductance can be obtained with a small number of turns of winding when a balancer coil is structured by using an adequately large core and coil, the parasitic capacitance between windings becomes smaller, thereby making self-resonance frequency higher. Therefore, even when making an excessive setting for the inductance having a shunt effect, the balancer coil sometimes has the effect with no difficulty. That is, when a balancer coil is structured with a large core, the range in which the balancer coil has shunt/balance effects is wide. Lighting experiments using such a large enough balancer coil have often been conducted.
A balancer coil for the use of cold-cathode florescent lamps is required to be small or flat, which makes the range in which the balancer coil has shunt/balance effects narrower. That is, each of small, flat, slim cores and the like makes effective magnetic permeability lower. As a result, multiple very thin copper wires have to be wound up.
Since a balancer coil for the use of cold-cathode florescent lamps, which is high in voltage and impedance, requires a large inductance, the number of turns of winding becomes larger. At the same time, this is accompanied by the fact that the parasitic capacitance between windings becomes larger, thereby making self-resonance frequency lower.
When self-resonance frequency becomes too low, the balancer coil loses shunt/balance effects. Therefore, a balancer coil especially for the use of cold-cathode florescent lamps should not be wound up excessively, and an excessive setting of inductance suppresses shunt/balance effects contrary to expectation.
In this respect, in order to obtain stable shunt/balance effects in a downsized balancer coil, as disclosed in the specification of Japanese Patent Application No. 2004-3740 (U.S. Patent Application No. 2004-155596), it is essential that an inductance value be set in an appropriate range by controlling the negative resistance characteristic of a cold-cathode florescent lamp.
As described above, the self-resonance frequency of a balancer coil is a factor which suppresses the downsizing of the balancer coil for cold-cathode florescent lamps.
FIG. 19 shows an example of a conventional balancer coil for cold-cathode florescent lamps, which has been smallest. Indicating the characteristics of the balancer, the inductance value of each coil is 200 mH, and self-resonance frequency is about 60 kHz as shown in FIG. 24. Each coil is layer-wound, thereby making self-resonance frequency low.
This is almost the limit value for a balancer coil for cold-cathode florescent lamps. Shunt/balance effects are sometimes shown for the use of a liquid crystal backlight panel, but the balance is sometimes lost suddenly.
Therefore, as disclose in the specification of Chinese (TAIWAN) Patent No. 521947, a ballast capacitor for being inserted into each cold-cathode florescent lamp in series is required so as to secure stability when the balance is lost.
In the example disclosed in FIG. 24, clearly, inductance increases when increasing the number of turns of winding of each balancer coil so as to secure shunt/balance effects, but the self-resonance frequency of the coil becomes further lower on the contrary, thereby losing the shunt/balance effects.
In this case, unless self-resonance frequency can be made higher, the balancer coil cannot be further downsized. Solutions to this problem include the example disclosed in the specification of Japanese Patent Application No. 2004-3740 (U.S. Patent application No. 2004-155596), wherein techniques relating to the downsizing of a balancer coil for cold-cathode florescent lamps are mainly used intensively.
Many approaches to practical application include the example in which balancer coils are laid out on the high-voltage side and on the inverter circuit substrate. This example, however, severely damages the balance effect.
The high-voltage-side wiring from a balancer coil up to a cold-cathode florescent lamp, which is particularly sensitive to the balance of a lamp current, should not be laid out on the inverter circuit side from the cold-cathode florescent lamp through the long wiring. Unless the high-voltage-side wiring is laid out adjacent to the cold-cathode florescent lamp as an independent shunt circuit module on the substrate, the effect thereof cannot be exhibited.
For the application of a backlight for a liquid crystal display television, multiple cold-cathode florescent lamps have to be lighted as a proposition. The inventors of the present invention then disclosed the connection method in FIG. 4 of the specification of Japanese Patent Application No. 2004-3740 (U.S. Patent application No. 2004-155596). In this connection method, however, a circulating current flows thereby decreasing the performance unless the leakage inductance of a balancer coil is made large. In this respect, the leakage inductance has to be larger.
In FIG. 6 of the specification of Japanese Patent Application No. 2004-3740 (U.S. Patent application No. 2004-155596) corresponding to FIG. 21, the technique is disclosed that magnetic fluxes of three coils or more are made to face one another so as to achieve the balance. Although this method has sufficient shunt/balance effects with three or four lamps, lamps of a number exceeding four decrease each coupling coefficient between windings as the number of lamps increases, thereby decreasing mutual inductance effective for shunting/balancing so that shunt/balance effects are gradually lost.
FIG. 6 of Japanese Laid-Open Patent Publication (Kokai) No. 2003-31383 (U.S. Pat. No. 6,717,372B2) discloses that balance/shunt effects can be obtained by windings W1 to Wn wound up around a single core.
This is visually shown as the structure in FIG. 25, which is difficult to achieve practically. There is no balance effect in spite of trying to balance a large number of lamps. Actually, shunt effects are maintained only by the self-inductance of each coil.    Patent document 1: Japanese Laid-Open Patent Publication (Kokai) No. Hei 7-45393    Patent document 2: Japanese Laid-Open Patent Publication (Kokai) No. 2003-31383 (U.S. Pat. No. 6,717,372B2)    Patent document 3: Japanese Patent Application No. 2004-3740 (U.S. Patent Application Publication No. 2004-155596)    Patent document 4: Japanese Patent Application No. 2004-79571 (U.S. patent application Ser. No. 11/081,545)    Patent document 5: Japanese Patent No. 3291852    Patent document 6: Chinese (TAIWAN) Patent No. 521947 Specification